1. Field of the Invention
This invention relates to a movement and/or positioning device with pentaxial movement and/or positioning of an object.
2. State of the Prior Art
This movement and/or positioning device can be employed especially with machines which move an object in five degrees of freedom, such as for example machine tools and machines for assembly and handling.
Known movement and/or positioning devices often use arrangements of a number of interconnected movement platforms, built up on one another and with a serial structure of corresponding movement axes of the movement platforms. This type of serial arrangement is realised, for example, with conventional machine tools with two movement platforms, whereby the drive axes of such a conventional machine tool are established one on the other. This means that the first axis bears (part of) the following second axis which is usually arranged orthogonal to the first axis.
In these types of movement and/or positioning devices with serial arrangements it follows that normally five independent drives are required for the realisation of pentaxial movements.
Taking the above reasoning into consideration, it is necessary with serial arrangements when moving or positioning an object to move the combined mass of all preceding movement platforms. Applied to the individual drives of the movement platforms, this means that the largest mass must be moved in each case by the first drives of a drive chain and that the individual drives of a drive chain in each case move different masses. Accordingly, this sort of movement and/or positioning device exhibits low dynamic characteristics for movement or positioning. In addition, any movement or positioning errors which occur are summed due to the coupled movement of the movement platforms with respect to one another during the movement and/or positioning. Furthermore, this sort of serial arrangement exhibits a low number of components of the same type, i.e. few repeated components, enabling only a restricted degree of modularisation.
Parallel kinematics offer an alternative. In this respect, this includes movement and/or positioning devices, generally termed hexapods, which exhibit six guide chains, which are also termed struts, each with five degrees of freedom at the joints, whereby normally rotation of the struts (guide chains) about their own longitudinal axis is prevented. Accordingly, the struts exhibit in each case a joint with three degrees of freedom and a further joint with two degrees of freedom, whereby one joint per guide chain is implemented as a drive. The drive of the guide chains is formed as a rotary or thrust drive which is supported in a frame or is integrated into the guide chain.
Consequently, in terms of gear systems this type of hexapod has six degrees of freedom of movement. Since a body to be moved or positioned always has six degrees of freedom, six drives are also needed to influence all six degrees of freedom. Most machine tools however only need five controllable degrees of freedom. This occurs, for example, with a milling machine in which the sixth degree of freedom is represented by rotation about the axis of the milling spindle.
One disadvantage with known hexapods is therefore that six drives must be used to generate five controllable degrees of freedom.
A further disadvantage with hexapods is the working space which is severely restricted due to the six guide chains, the said working space being determined by the boundaries of the swivel angles on an object or object holder to be moved. In principle, as the number of implemented guide chains increases, i.e. the total number of struts used which connect an object or an object holder to the frame, the available working space becomes smaller, because greater regions of collision between the guide chains restrict this working space.
Furthermore, arrangements are known for producing pentaxial movements with five drives which are arranged mixed parallel and serial. Here, the suggestion is made that three guide chains (termed struts in the following), variable in length, are arranged in parallel between a frame and an object holder. With the associated drives movement over all five axes is realised, whereby these axes, however, cannot be controlled separately. To compensate for the necessarily occurring swivel movements, the object holder exhibits two further serially arranged swivel axes.
A disadvantage of this type of arrangement is that the serially arranged swivel axes, which must be active in the space for all movements, limit the dynamic characteristics of the movement and/or positioning device. In addition, the increased mass is disadvantageous, which the drives of the struts, varying in length, must move.
In U.S. Pat. No. 4,569,627 a movement and/or positioning device is suggested in which exclusively three struts for linking the object holder to the frame are used. These three struts can be controlled in their length. In addition, two of the three struts can execute a torsional movement.
A significant disadvantage of this movement and/or positioning device is the stress on the struts in the torsion or bending direction. Since the rods formed as struts exhibit a high longitudinal stiffness, but are very compliant in the torsion and bending direction, this type of movement and/or positioning device exhibits a limited overall stiffness and a limited dynamic performance.